1. Field of the Invention
The present invention relates to a magnetic bearing type vacuum pump, and more particularly, to a turbo molecular pump used for exhausting gas in a semiconductor manufacturing apparatus.
2. Description of the Related Art
With the recent abrupt innovation of industrial and scientific technologies, a demand on a vacuum apparatus realizing a high vacuum environment has been increased.
For this reason, various vacuum pumps have been developed to be adopted for respective purposes. Requirements for various performances to be possessed in these vacuum pumps, such as an exhaust quantity per unit time, a vacuum degree to be reached, and suppression of vibrations, have also been made severe from year to year.
Since a magnetic bearing type turbo molecular pump meets these various performance and is highly reliable, it is widely used in exhaust for a semiconductor manufacturing apparatus, in exhaust for a vacuum pump of a scientific and scientific technique study.
The magnetic bearing type turbo molecular pump is designed to magnetically float a shaft by using electromagnets, and to maintain the shaft in a levitated state in a non-contact manner. Therefore, a lubricating oil is not required in bearing portions, and thus there is no fear that the lubricating oil gets mixed in the inside of the vacuum apparatus. Further, it is almost completely free from problems of friction, wearing, noise, etc., and further it can rotate the shaft at a high speed.
Hereafter, a structure of the magnetic bearing type turbo molecular pump will be discussed briefly.
The magnetic bearing type turbo molecular pump is composed of a substantially cylindrical casing, a stator and a rotor housed in the casing, etc.
The stator is formed in an inner circumferential surface of the casing, which has a plurality of stator blades arranged in multiple stages and extending toward a center direction of the casing.
A shaft is rotatably held on a central axis of the casing. The shaft is axis-supported by magnetic bearings fixed to the stator so as to be rotatable at a high speed by a motor portion formed in the substantially central portion of the shaft.
A suction port is formed in one end of the casing, and a rotor is attached to a suction port side portion of the shaft.
A plurality of radially arranged rotor blades are attached around the rotor in multiple stages. The stages of the rotor blades and the stages of the stator blades are alternately positioned.
When the shaft is rotated at a high speed by the motor, the rotor blades are rotated with this rotation. When the rotor blades are rotated, by the action of the rotor blades and the stator blades, the turbo molecular pump sucks gas through the suction port, and exhausts the gas through an exhaust port formed in the casing.
FIG. 4 shows a magnetic bearing portion, a shaft, and their peripheral portion in a conventional magnetic bearing type turbo molecular pump.
The magnetic bearing portion is composed of four electromagnets 130 disposed around a shaft 111 with predetermined clearances therebetween.
The electromagnets 130 are distributed at an angular interval of 90 degrees around the shaft 111 to be confronted therewith. On the other hand, a portion of the shaft confronted with the electromagnets 130 is formed into an electromagnet target 129. The electromagnet target 129 is constructed such that ring-like steel plates, the surfaces of which are made insulated, are passed through and fixed to the shaft 111.
The electromagnets 130 attract the electromagnet target 129 by magnetic force to magnetically float or levitate the shaft 111 in the radial direction and to hold it in a non-contact manner.
Since laminated steel plates composing the electromagnet target 129 are insulated one from another, eddy currents induced on the surface of the shaft 111 by magnetic fields generated by the electromagnets 130 cannot flow across the plurality of laminated steel plates.
For this reason, the loss of eddy currents, the heat generated on the shaft 111 by the eddy currents, and the like can be suppressed.
A coil 128 of a radial sensor is provided in the vicinity of the magnetic bearing portion and spaced from the shaft 111 at a predetermined clearance. On the other hand, a surface of the shaft 111 confronted with the coil 128 is formed into a radial sensor target 136 composed of laminated steel plates.
The coil 128 is a part of an oscillating circuit formed in a control portion installed outside the turbo molecular pump.
When the oscillating circuit is oscillated, a high frequency current flows through the coil 128 with the oscillation so that the coil 128 generates a high frequency magnetic field on the shaft 111.
If a distance between the coil 128 and the radial sensor target 136 is varied, the oscillating width of the oscillator is varied and thus the displacement of the shaft 111 can be detected.
A collar 135, made of metal, such as a stainless steel, is formed between the radial sensor target 136 of the shaft 111 and the electromagnet target 129 thereof. Since the collar 135 and the electromagnets 130 are located closely, the magnetic field 134 generated when the electromagnets 130 is excited can also permeate through the collar 135.
However, since the collar 135 does not have eddy current preventing means made, for instance, by laminated steel plates, not only the collar 135 is heated by the eddy current generated by the magnetic field 134, but also there is wasteful electric power consumption due to the loss of the eddy current. Further, if the eddy current is generated in the collar 135, this eddy current applies, onto the shaft 111, a force in a direction braking the rotation.
Accordingly, an object of the present invention is to provide a magnetic bearing type vacuum pump, which can suppress the generation of the eddy current on the shaft to be small in heat generation on the shaft and to be excellent in efficiency.
According to the present invention, in order to attain the above objects, there is provided a magnetic bearing type vacuum pump characterized by comprising: a substantially cylindrical casing formed with a suction port and an exhaust port; a stator formed within the casing; a shaft axis-supported by the stator to be rotatable; a rotor attached to the shaft to be rotated together with the shaft; a motor for driving and rotating the shaft; a magnetic bearing portion including a plurality of electromagnets disposed around a predetermined portion of the shaft with a predetermined gap from the shaft, and an electromagnet target formed on a portion of the shaft confronted with the electromagnets; a radial sensor portion disposed in the vicinity of the magnetic bearing portion, and including a radial sensor disposed around the shaft with a predetermined gap therefrom, and a radial sensor target formed on a portion of the shaft confronted with the radial sensor; and a thrust bearing portion for holding the shaft in a thrust direction, in which laminated steel plates laminated through insulation are formed on the shaft between the electromagnet target and the radial sensor target (first structure).
The laminated steel plate portion in a first structure can include the steel plates laminated in an axial direction of the shaft (second structure).
The electromagnet target and the radial sensor target in a first or a second structure can be formed of laminated steel plates, and the laminated steel plate portion can be integrally formed with the electromagnet target and the radial sensor target (third structure).
Further, the steel plate in any one of first to third structures can include a silicon steel plate (forth structure).
Also, the magnetic bearing pump in any one of first to forth structures can be composed of a turbo molecular pump having rotor blades on the rotor and stator blades on the stator, a threading groove type pump with threading groove formed on at least one of the rotor and the stator, or a magnetic bearing type vacuum pump having the turbo molecular pump in the suction portion side and the threading groove type pump in the exhaust port side.